1. Field of the Invention
The present invention is related to amplifier systems, and more particularly, to radio-frequency amplifier systems.
2. Description of the Related Art
Radio frequency ("RF") amplifier circuits for use in RF transmitters are known. Conventional RF amplifier circuits generate RF signals and may be coupled with an antenna so that the antenna can transmit the RF signals. Conventional RF amplifier circuits typically use class C amplifiers to obtain maximum efficiency. Efficiency is defined by the ratio of output power to the supplied power. Class C amplifiers are theoretically 100% efficient and are, therefore, popular for use in products where power consumption and battery longevity is an important criteria, such as in wireless products.
FIG. 1 is a circuit diagram illustrating a conventional class C amplifier 101. A class C amplifier typically includes a capacitance 110, a resistor 120, a transistor 130 having a base, a collector, and an emitter, a supply voltage 150, a ground, an output signal line 170, and an LC circuit 175 that includes an inductor 175a and a capacitor 175b. An input voltage source 105 with a source resistance 105a provides the RF signal to the amplifier.
The input voltage source 105, with its source resistor 105a is coupled to the capacitance 110 and the ground 160. The capacitance 110 is coupled to the resistance 120 and the base of the transistor 130. The resistance 120 is coupled to the base of the transistor 130 and the ground 160. The emitter of the transistor 130 is coupled to ground 160 and the collector of the transistor 130 is coupled to the LC circuit 175, which is coupled to the supply voltage 150. An amplified signal is output through the output signal line 170 that is coupled to the collector of the transistor 130.
During operation of the conventional amplifier circuit 101, the input voltage source 105 is typically a sinusoidal input of 3.0 volts or greater, peak-to-peak. The resistance 120 is used to adjust the conduction angle of the transistor. Due to the non-linearity of the base-emitter junction of the transistor and capacitor 110, the transistor is automatically negative-biased, and therefore, not amplifying. The base resistor 120 reduces this negative bias during each input signal period and opens the transistor for a small amount of time each period. This time, expressed in degrees, called a conduction angle. The transistor works only during the conduction angle.
For low voltage applications (typically 3 volts or less), where the input signal is also low, this self-biasing process could not happen properly. First, the input signal could be too low (lower than 1.4 volt peak-to-peak ("V.sub.pp ")) for exceeding the base-emitter junction voltage (V.sub.be =0.7 volts) of the transistor, and therefore, is unable to open the transistor. Second, even if the input signal was slightly higher, and therefore, able to exceed the V.sub.be voltage, the resulting negative bias would be too low. The base resistor must be very low to correctly adjust the conduction angle of the transistor; this however, has the effect of dramatically decreasing the performance of the class C amplifier. Thus, class C amplifiers are not suitable for low voltage designs and applications.
A wireless product, for example an RF wireless keyboard, requires an increased transmission range in order to realize its wireless advantages. Therefore, the radiation power of the amplified signal must be strong enough to provide these advantages. Moreover, a wireless RF product runs on batteries and requires low power consumption to increase battery lifetime. Conventional amplifiers (class A) which are able to work at low voltages, however, are not optimal for such applications because of their low efficiency. That is, only a small portion of the energy drawn from the battery is provided to an antenna of a wireless RF keyboard, while the remaining portion is wasted.
Therefore, there is a need for (1) a high efficiency RF amplifier system that (2) provides adequate signal strength for longer transmission ranges, but (3) that has low power consumption while (4) being low cost to achieve and (5) works at low supply voltage.